Electrical submersible pumps (“ESP”) are used for pumping high volumes of well fluid, particularly in wells requiring artificial lift. The ESP typically has at least one electrical motor that normally is a three-phase, AC motor. The motor drives a centrifugal pump that may contain a plurality of stages, each stage comprising an impeller and a diffuser that increases the pressure of the well fluid. The motor is filled with a dielectric lubricant or oil that provides lubrication and aids in the removal of heat from the motor during operation of the ESP. A seal section is typically located between the pump and the motor for equalizing the pressure of the lubricant contained within the motor with the hydrostatic pressure of the well fluid on the exterior. The seal section is filled with oil that communicates with the oil in the motor.
The ESP is typically run within the well with a workover rig. The ESP is run on the lower end of a string of production tubing. Once in place, the ESP may be energized to begin producing well fluid that is discharged into the production string for pumping to the surface.
During operation, the temperature of the oil in the motor of the ESP increases due to friction in the motor, causing the volume of the oil to also expand. The oil is vital to maintaining the motor within its rated temperature and maintain reliability. However, oil may migrate outside of the motor when it expands, resulting in less oil for protecting the motor and possible contamination of other parts of the ESP.
To counteract the expansion of the oil, a bladder, bellows or labyrinth seals form an expansion chamber within a seal section of the ESP. The internal expansion chamber provides additional volume into which the oil can expand. However, this requires increasing the length of the ESP system, which can be a problem for a sea floor booster pump. In addition, the internal expansion chamber may fail and the entire ESP system would need to be replaced. This could result in costly downtime.
A technique is desired to allow for expansion of the motor oil surrounding the motor that may translate to extended life and increased reliability of the motor without increased ESP length.